Electroacoustic device



E. E. MOTT ELECTROACOUSTIC DEVICE July 15, 1941.

Filed May 19, 1939 2 Sheets-Sheet 1 F /G. 24 22 26 33 33 29 34 22 2oFIG. 6

woo/1L CIRCLE lNl ENTOR By E. E. MOTT ATTORNEY I July 15, 1 941. E. E.MOTT 2,249,161

' ELECTROACOUSTIC DEVICE Filed May 19, 1939 2 Sheets-Sheet 2 INVENTOR E.5. M07 7' I l msaus/vcy a; G

ATTORNEY Patented July 15, 1941 ELECTROACOUSTIC DEVICE Edward E. Mott,Upper Montolair, N. .L, asslgnor to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York Application May19, 1939, Serial No. 274,484

12 Claims.

This invention relates to electroacoustic devices and more particularlyto telephone receivers having a magnetic vibratile element or diaphragm.

-One object of this invention is to reduce the number of component partsof an electroacoustic device and thereby to simplify the structure,reduce its size and weight, and facilitate the manufacture thereof.

Another object of this invention is to improve the efliciency ofelectroacoustic devices and more particularly to increase the efiiciencyof the magnetic system of telephone receivers.

A further object of this invention is to im prove the vibratingcharacteristics of a magnetic diaphragm in telephone receivers and, morespecifically, to substantially eliminate Vibration of such a diaphragmin other than its fundamen tal mode.

In one illustrative embodiment of this invention, a telephone receivercomprises a magnetic diaphragm, which may be clamped or unclamped at itsperiphery, a support therefor, and actuating means, includingpole-pieces and a magnet, for the diaphragm.

In accordance with one feature of this invention, the magnetic systemcomprises a pair of pole-pieces having parallel arms and also extensionsor lugs affixed to one face of the support, and a single bar magnethaving a high ratio of coercive force to remanence mounted between theparallel arms of the pole-pieces and aifixed thereto.

In accordance with another feature of this invention, the pole-piecesare so dimensioned and related that the driving forces are applied tothe diaphragm at the first nodal circle thereof whereby the diaphragmvibrates substantially only in its fundamental mode and distortionthrough harmonics is prevented.

The invention and the foregoing and other features thereof will beunderstood more clearly and fully from the following detaileddescription with reference to the accompanying drawings in which:

Fig. 1 is a side view in section of a telephone receiver illustrative ofone embodiment of this invention;

Fig. 2 is a top view of the receiver shown in Fig. 1 with the covermember and diaphragm removed;

Fig. 3 is a perspective view of the pole-piece and magnet structureembodied in the telephone receiver shown in Figs. 1 and 2;

Fig. l is a diagram illustrating the bending curve of a diaphragm freelysupported at its periphery, as in the receiver shown in Fig. 1;

Fig. 5 is a diagram illustrating the bending curve of a similardiaphragm clamped at its periphery;

Fig. 6 is a diagrammatic View illustrating the relation of thepole-pieces with respect to the first nodal circle of a diaphragm, as inthe telephone receiver illustrated in Fig. 1;

Fig. 7 is a side view in section or another receiver wherein thediaphragm is driven at its first nodal circle;

Figs. 8 and 9 are top and perspective views, respectively, illustratinganother form of magnetic structure which may be employed to drive adiaphragm at its first nodal circle;

Fig. 10 is a graph showing comparative response characteristics forsimilar diaphragms driven at points at and removed from its first nodalcircle;

Fig. 11 is a side view in section of a telephone receiver illustrativeof another embodiment of this invention; and

Fig. 12 is a bottom View of the receiver shown in Fig. 11.

Referring now to the drawings, the telephone receiver shown in Figs. 1and 2 comprises a centrally apertured foundation frame or support It,such as a disc, of non-magnetic material, such as an aluminum or zincalloy, having on one face thereof an annular ridge or seat It. Aliixedto the frame l5 and extending over the central aperture therein is aplate ll of non-magnetic material, such as phenol fibre, having a pairof parallel elongated slots therein. A cross-piece 18, which may beintegral with the frame or made separately and fitted in the centralaperture therein, extends across the central aperture in the frame andforms therewith and the plate H a pair of pockets in which the signalcoils ii! are positioned.

Seated upon the annular ridge I6 is a diaphragm 20 which may be acircular disc of magnetic material. A cover member 2|, having apertures22 allowing egress of sound waves, overlies the diaphragm 20 and isclamped to the frame [6 by an annular crimped band 23. The cover member2| is provided with an annular ridge 24 slightly spaced from theperiphery of the diaphragm, as through the intermediary of an annularspacer 25 between the cover member 2| and the frame I5, so that thediaphragm is held in position solely by magnetic attraction.Alternatively, the diaphragm may be clamped at its periphery between theridges IB and 24.

The cover member 2| may have mounted in its central portion an auxiliarypermanent magnet 26, as described fully in the application Serial No.274,485, filed May 19, 1939, of Louis A. Morrison.

The electromagnetic structure for actuating the diaphragm 20 includes apair of pole-pieces, designated generally as 21, and a single barpermanent magnet 28 magnetized to have poles of opposite polarity at theends of its shorter axis. As shown clearly in Fig. 3, each of thepole-pieces 2! includes a T-shaped portion having an upright arm 29 anda lateral arm 30, and mounting flanges or lugs 3|. The permanent magnet28 is disposed between the arms 30 and aiiixed thereto as by soldering.As shown clearly in Fig. 1, the pole-pieces 21 are affixed to the frame[5, as by screws 32 extending through the flanges or lugs 31 andthreaded into the frame, with the T-shaped portions thereof parallel andthe arms 29 passing through the slots in the plate II.

It has been determined that the relative dimensions of a magnetrequisite for producing a desired flux are determinable from theequation The magnet 28 is of an alloy of 53 per cent iron, 20 per centnickel, 10 per cent aluminum, 12 per cent cobalt, and 5 per cent copper.A magnet of this composition, it has been found, has a very highcoercive force in proportion to the remanence and enables the use of avery short bar magnet. For example, in the receiver shown in Fig. 1, amagnet of a length of inch and section of A; inch by inch may be usedsatisfactorily. In a similar structure including a magnet of cobaltsteel, two magnets 1 inches long by ,4; inch square would be necessaryto produce a comparable flux. The decrease in the magnet length thusresults in a short magnetic the opposite faces of the frame [5. Thisresults in a very thin and lightweight structure which is especiallysuitable for use in headsets such as used, for example, by telephoneoperators.

The spaces between the coils and the frame [5 and cross-member i 8 maybe sealed with thermoplastic material 33 whereby a shallow chamber 34below the diaphragm 20 is formed. This chamber may be completely closedexcept for a restricted aperture 35 having a sheet of acousticresistance material 35, such as silk, extending thereacross. Theelements of the acoustic network thus formed may be designed inaccordance with the invention disclosed in the application Serial No.93,792, filed August 1, 1936, of Louis A. Morrison and Edward E. Mott toobtain a uniform response characteristic throughout a wide frequencyrange.

In receivers of the general type disclosed, it has been found that themotionof the diaphragm is characterized by two predominant modes ofvibration, namely, a piston-like mode, termed the fundamental, and aharmonic in which the central and outer portions of the diaphragmvibrate in opposite phase. The bending curves of a diaphragm, such asthe diaphragm 2B, unclamped at its periphery are illustrated in Fig. 4,where curve F illustrates the bending for the fundamental mode and curveH illustrates the bending for the harmonic mode. Fig. 5 shows similarbending curves for a similar diaphragm clamped at its periphery.

The effect of the harmonic mode of vibration is to introduce a resonancepeak in the response of the receiver. Thus, as illustrated by the curveA in Fig. 10, the fundamental mode of vibration produces a peak B in theresponse and the harmonic resonance for example produces a second peak0. The peak B may be suppressed by damping, as described in theaforementioned application of Louis A. Morrison and Edward E. Mott; thepeak C, however, cannot be suppressed by acoustic damping because of therotating character of the resonance.

In the case of a diaphragm unclamped at its periphery, it has beendetermined that the harmonic resonance produces a peak C at 6.7 timesthe fundamental frequency. For a clamped diaphragm, the harmonicresonance occurs at a frequency 3.9 times the fundamental.

It will be noted from Figs, 4 and 5 that the harmonic mode produces anodal area passing through points X at which there is no motion. If adriving force were applied to the diaphragm at this area, no motionwould occur at any point along the diaphragm on the curve H because ofthe principle of reciprocity. In other words, resonance due to anyharmonic mode of vibration may be suppressed by driving the diaphragm atthe nodal circle corresponding to this particular mode. In a telephonereceiver, it has been found satisfactory to drive the diaphragm at itsfirst nodal circle, the resonance frequencies for higher harmonics lyingoutside of the range of frequencies it is usually desired to transmit.

It has been determined that for a diaphragm, such as the diaphragm 20,unclamped at its periphery, the first nodal circle has an averagediameter 44.5 per cent of the seating diameter and that for a diaphragmclamped at its periphery, the average diameter of the first nodal circleis 37.9 per cent of the clamping diameter.

In a bipolar receiver of the construction shown in Fig. l, suppressionof the resonance due to the harmonic corresponding to the first nodalcircle may be effected by spacing the arms 29 of the pole-pieces sothat, as shown in Fig. 6, the pole faces lie on the nodal circle and areapproximately bisected by it. In this case, as indicated by the curve Ain Fig. 10, the fundamental peak B is shifted slightly to B and the peakC is entirely eliminated.

Various other forms of magnetic structures may be employed to effectapplication of the driving forces to the diaphragm at the nodal circle.For example, as shown in Fig. 7, the magnet structure may includeconcentric annular pole-pieces 31 and 38, the inner pole-piece 31 havinga diameter equal to the average diameter of the nodal circle of thediaphragm 20. In the form shown in Figs. 8 and 9, the polepieces 39 maybe U-shaped, mounted at opposite ends of a bar magnet 40 and spaced sothat the pole faces thereof lie opposite equally spaced areas of thenodal circle. In the forms shown in Figs. 6, 8 and 9, the pole-piecesmay be arouate and of the same curvature as the nodal circle to providegreater conformity between the pole faces and the nodal circle.

In the receiver illustrated in Figs. 11 and 12, which is generallysimilar to that shown in Figs. 1 to 3 and described heretofore, theframe I is provided with an inwardly extending flange 4| upon which thecoils l9 are seated and into which the pole-piece mounting screws 32 arethreaded. A suitable terminal block 42 of insulating material ispositioned in a recess or cavity in the frame I5 and is secured to theframe I5 by a screw 43. The spaces between the coils I9 and the framemay be sealed by thermoplastic material 33. The cover member 2| isprovided with a peripheral shoulder 54 which spaces the cover from thediaphragm 2% the diaphragm being held in position solely by magneticattraction. Alternatively, the diaphragm may be clamped at itsperiphery,

Although specific embodiments of the invention have been shown anddescribed, it will be understood, of course, that they are butillustrative and that various modifications may be made therein withoutdeparting from the scope and spirit of this invention as defined in theappended claims.

What is claimed is:

l. A telephone receiver comprising a substantially flat foundationmember having an annular seating surface on one face thereof and havingalso a central aperture, a magnetic diaphragm mounted on said seatingsurface, and actuating means for said diaphragm including a pair ofsubstantially parallel T-shaped polepieces the legs of which extendthrough said central aperture and terminate in proximity to saiddiaphragm and the arms of which extend along the opposite face of saidfoundation member, said pole-pieces having lugs seated upon and securedto said opposite face, signal coils on said legs and a single barpermanent magnet extending between and secured to said arms of saidpole-pieces.

2. A telephone receiver in accordance with claim 1 wherein said magnetis a rectangular bar having its shorter dimension normal to said armsand is of an alloy of 53 per cent iron, 10 per cent aluminum, 20 percent nickel, 12 per cent cobalt, and 5 per cent copper.

3. A telephone receiver comprising a substantially flat support having aseating surface on one face thereof and having also a central aperture,a diaphragm seated on said surface, and means for actuating saiddiaphragm including a pair of pole-pieces afiixed to the opposite faceof said support and having pole tip portions extending through saidaperture and other portions extending beyond said opposite face, a barpermanent magnet between and secured to said other portions and signalcoils encompassing said pole tip portions and positioned entirely Withinsaid aperture.

4. A telephone receiver comprising a disc frame having an aperturetherein and an annular seat on one face thereof, a diaphragm seated onsaid seat, pole-pieces having arms extending through said aperture andterminating adjacent said diaphragm, signal coils encompassing said armsand located entirely within said aperture, and filler means sealing thespaces between said coils and said frame whereby a shallow acousticchamber is formed between said diaphragm and said frame, said chamberbeing completely closed except for a restricted opening through saidframe.

5. A telephone receiver comprising a substantially fiat frame having acentral aperture, an inwardly extending flange adjacent one face and aseating surface on the opposite face, a diaphragm seated on saidsurface, a magnetic structure including pole-pieces extending throughsaid aperture and terminating adjacent said diaphragm, and signal coilsseated upon said flange and encompassing said pole-pieces.

6. A telephone receiver in accordance with claim 5 wherein saidpole-pieces have parallel portions extending outwardly beyond said onesurface of said frame and comprising a bar permanent magnet securedbetween said portions.

7. A telephone receiver comprising a disc magnetic diaphragm, and meansfor actuating said diaphragm comprising a pole-piece having itspole-face in juxtaposition to the first nodal circle of said diaphragm.

8. A telephone receiver comprising a magnetic disc diaphragm, and meansfor actuating said diaphragm including a plurality of pole-piecesterminating in juxtaposition to one surface of said diaphragm at areassubstantially coincident with the first nodal circle of said diaphragm.

9. A telephone receiver comprising a magnetic disc diaphragm, and meansfor actuating said diaphragm including a pair of parallel polepiecesterminating adjacent the first nodal circle of said diaphragm and havingtheir pole-faces substantially bisected by said circle.

10. A telephone receiver comprising a magnetic disc diaphragm freelysupported at its periphery, and means for applying driving forces tosaid diaphragm at areas thereof on and immediately adjacent a circlehaving a diameter of substantially 44.5 per cent the diameter of saiddiaphragm.

11. A telephone receiver comprising a magnetic disc diaphragm clamped atits periphery, and means for applying driving forces to said diaphragmat areas thereof on and immediately adjacent a circle having a diameterof substantially 37.9 per cent of the diameter of said diaphragm.

12. A telephone receiver comprising a substantially flat frame having aseating surface on one face thereof, a magnetic disc diaphragm seated onsaid surface, and means for applying driving forces to said diaphragmalong the first nodal circle thereof including a pair of polepieceshaving parallel arms projecting through said frame and terminatingadjacent said diaphragm and having also mounting lugs seated upon theopposite surface of said frame, and a bar magnet secured between saidarms.

EDWARD E. MOT'I'.

